1. Field of the Invention
This invention relates to fabrication of semiconductor devices, Micro Electro Mechanical Systems (MEMS) devices, and more specifically to wafer bonding in fabrication of these devices.
2. Background
Wafer-to-wafer bonding is widely used in fabrication of different MEMS devices including sensors, actuators and micromechanical structures.
There are several types of bonding process known in the art (see “Three-Dimensional Bonding Technologies for MEMS”, Vladimir Vaganov, Nickolai Belov, Sebastiaan in't Hout presented at SPIE 2003): fusion bonding, bonding with plasma assisted surface activation, anodic bonding, frit glass bonding, metal bonding, deformation welding, soldering, eutectic bonding, and a wide group of bonding processes that can be referred as polymer or adhesive bonding. Polymer bonding utilizes a wide spectrum of bonding materials that have strong adhesion to material of the wafers, typically, silicon, or to a surface layers formed on the wafers
Selection of the bonding method and material is often dictated by the nature of the device and application. Besides general requirements to the bonding, like high bond strength, chemical stability of the bond, environmental protection of the encapsulated structure, long-term stability, thermal budget, etc.
Additional specific requirements include:                Bonding should be uniform across the wafer and reproducible from wafer to wafer, independently on wafer thickness, total thickness variation, bow, non-flatness, non-parallelism, profile formed on the wafers, thickness and non-uniformity of the bonding layer across the wafer, its mechanical parameters, bonding area and other variables.        Often bonding material should have low Young modulus for minimizing the induced or residual stresses in the wafers after bonding.        In some cases there might be additional requirements to electrical or thermal conductivity of the bonding material.        
For all bonding processes, which require application of an external mechanical load, a threshold level of mechanical stress at the bonding surfaces is required. This threshold stress σt often in combination with temperature initiates, activates and stimulates the successful flow of the bonding process. Usually the higher the stress above the threshold—the shorter the process and more uniform the bonding is. However, the downside is the higher level of residual stress in the microstructures induced by larger force and higher temperature. Therefore the applied load during the bonding should be optimized: minimized to avoid damage and residual stress, on one hand, while providing a required stress exceeding the threshold, on the other hand.
One of the conventional assumptions of “ideal” bonding process is that the total thickness variation (TTV) and the surface roughness of the bonding wafers are small enough for a sufficiently good bond. This assumption results in conclusion that required uniformity of mechanical stress in the bonding areas across the wafer is achieved. As shown below this is not the case.
FIG. 1 illustrates an example of the bonding, which conditions assumed “ideal”.
The external force 33 is applied by “flat” heated platens 6 and 8 to the outer surfaces 14 and 16 of the dummy substrates 5 and 7, which are also assumed to be ideally flat. Heated platens 6 and 8 are ideally parallel to each other and dummy substrates 5 and 7 are also ideally flat and have constant thickness. The force is transferred through these dummy substrates from the inner surface 18, 20 of the dummy substrates to the outer surfaces 22, 24 of the bonding wafers 2, 4 and through the thickness of the bonding wafers to the bonding surfaces 30, 32. If all the surfaces are ideally flat then external force will be uniformly distributed to the bonding areas across the whole wafer and as soon as the stress in the bonding areas reaches the threshold value (plus temperature contribution to the surface activation) the uniform bonding will occur.
In reality all the surfaces: platens, dummy substrates, bonding wafers are not ideally flat. It means that being brought in physical contact any two surfaces will contact each other in certain number of relatively small contact areas (or contact points) which distribute stresses across the two surfaces non-uniformly. In case of non-ideal surfaces the external force applied to platens 6 and 8 in FIG. 1 will be transferred not uniformly to the inner surfaces of the dummy wafers 5 and 7. For the same reason it would be randomly on the surface and unequally, corresponding to profile of the wafer stack due to TTV distributions and roughness of two contacting surfaces, transferred to the bonding areas 30 and 32 of the bonding wafers. Of course, with some limitations, by increasing the external force one can achieve the required bonding conditions in all bonding areas. However, that comes with a price, the quality of bonding will be different in different wafer areas and induced residual stresses after bonding will be greater.
FIG. 2 illustrates an example of real conditions of the bonding. The external force 33 is applied by non-flat and “rough” surfaces 10, 12 of the heated platens 6, 8 to the outer surface 14, 16, which are also non-flat and rough, of the dummy substrates 5 and 7. These two pairs of surfaces are contacting each other in contact areas 9, 11, 13 and 19, 21 correspondingly. The force through these contact points is transferred to the inner surface of the dummy substrates, which is also non-flat and rough and contacting to the outer surfaces of the bonding wafers 2 and 4, which also have their own non-flatness and roughness creating together a different distribution of the physical contact points. In FIG. 2 the inner surfaces of the dummy wafers and the outer surfaces of the bonding wafers are skipped for simplicity. Only final non-flat and rough inner surfaces 26 and 28 of the bonding wafers 2 and 4 are shown in the center of FIG. 2. These two surfaces of the bonding wafers are contacting in the areas 15 and 17. These contact areas and their distribution across the wafers do not correlate with the position and layout of the bonding areas 29, 31 and 30, 32 on the surfaces 26 and 28 of the bonding wafers 2 and 4. It is obvious that through the thickness of the bonding wafers (and dummy wafers if they participate) the external force 33 will be randomly transferred to the bonding surfaces 26 and 28. In these conditions it is hard to expect a uniform and reproducible bonding in the areas 29, 31 and 30, 32 across the wafer and from wafer to wafer. This is a challenge, which different wafer bonding processes are facing in practice.
Another conventional assumption is that bonding wafers and other substrates participating in the bonding process are rigid to provide complete and uniform transferring of the external force applied by platens to the bonding surface. In reality it is not so. Silicon is an excellent elastic material and it is capable to transfer a local force applied to one surface of the wafer trough the thickness of the wafer to the other surface of the wafer non-uniformly. If two even ideally flat wafers 2 and 4 bring in physical contact and then apply a force 34, 35, 36 in discrete points on the top wafer 2, then the distribution of the stress 3 on the surface of wafers contact will look qualitatively, as shown in FIG. 3. The maximum stress σ will be achieved in the area close to points 37 38 and 39 at the contact surface, as shown in FIG. 3. Point 38 corresponds to zero coordinate on X-axis on the graph. The larger the distance along the contact surfaces of the wafers from the points 37, 38, 39 the lower the stress at the contact surfaces will be. At some points X1, X2, X3, X4, X5 and X6 the stress might drop below the threshold stress σt. It means that the areas between X1 and X2, between X3 and X4 and also between X5 and X6 would have enough stress for successful bonding. All the rest areas of the wafers would not be bonded if one would try to bond two wafers across their entire surfaces. This figure illustrates that elastic silicon can transfer a concentrated force locally into a certain areas.
This consideration along with understanding of the non-uniform transferring of the external force to the bonding surface creates the need for better and higher quality wafer-to-wafer bonding processes providing higher yields in fabrication of semiconductor and MEMS devices.